Yes, ATX standards are ATX standards, that case supports any ATX, mATX or mITX motherboard.
No, there's absolutely no issues with power supplies with this case, in fact the only cases that do have issues are non-ATX cases like SFF, SFX, some mITX etc, the really slim, small, tiny stuff. I've never heard of any ATX psu having any issues in an ATX mid-tower other than maybe blocking a bottom fan port which most don't use anyway.
I7-9700k is an 8 core/8 thread cpu. (think like an old i5 that's 4c/4t). The i9 9900k is an 8 core/16 thread cpu. (that's like the old i7 version of the i5 + hyperthreading). Is the i7 9700k good? Yep, sure is. It's the same as any other old i5, just one step down from the top of the line i7. Intel just bumped up the i numbers to differentiate from the 6c/6t i5 9600k with its 2 less cores.
The real question is how long will that last. Windows 10 and many games don't use more than 8Gb of ram, but quite a few do so 16Gb is universally recommended. Starwars will be releasing its latest game sooner or later. Minimum 8Gb, no surprise there, but recommended is now 32Gb of ram. Surprise. Just 6 short years ago, quad core cpus ruled budget oriented gaming. Now, about obsolete for any new title. 6c/6t cpus will be next. So just how long before 8c/8t start running into trouble with high % usage. The hyperthreaded 9900k will be absolutely the last cpu of recent generations to become obsolete. It's going to outlast them all. That's the difference.